1. Field of the Invention
The present invention relates to a developing roller used in, e.g., electrophotographic image forming apparatus such as copying machines and laser beam printers, and also relates to a process for its production, and a developing assembly and an image forming apparatus which make use of the developing roller.
2. Description of the Related Art
Electrophotographic image forming apparatus which employ a pressure development process are known as electrophotographic image forming apparatus such as copying machines, laser beam printers, and receiving sets of facsimile machines. The pressure development process is a development process in which a non-magnetic one-component developer is used as a developer and the developer is made to adhere to an electrostatic latent image held on a photosensitive drum, to render the latent image visible. This process is widely used because it requires no magnetic material, facilitates manufacture of simple and compact apparatus, and also facilitates preparation of color developers.
An electrophotographic image forming apparatus employing such a pressure development process has, as shown in FIG. 3, an image bearing member for holding thereon an electrostatic latent image such as a photosensitive drum 21 rotatable by a rotating mechanism, and, disposed around this photosensitive drum 21, a charging member 22 which makes the photosensitive drum 21 charged electrostatically. This electrophotographic image forming apparatus further has an exposure unit. It makes laser beam 23 irradiate the photosensitive drum 21 and thereby, the photosensitive drum 21 is discharged at the area where the laser beam 23 irradiates, and electrostatic latent images are formed on the photosensitive drum 21 which correspond to the images to be finally formed. This electrophotographic image forming apparatus still further has a developing assembly and a transfer roller 29. The developing assembly feeds a developer to the electrostatic latent images formed on the photosensitive drum 21, to perform development, and the transfer roller 29 transfers the developer having adhered to the electrostatic latent image, onto a transfer material (recording material). It still further has a cleaning blade 30 which destaticizes the surface of the photosensitive drum 21 from which the developer has been transferred and performs cleaning, and a fixing assembly 32 which fixes the developer held on the transfer material. The above developing assembly is provided with a developing roller 25, a developer feed roller 26 which coats the surface of the developing roller 25 with a developer 28 held in a developer container 34, and a developing blade 27 which adjusts to a more uniform thin layer the developer with which the surface of the developing roller 25 is kept coated. The developing roller 25 rotates while feeding to the photosensitive drum 21 the developer made uniform by the aid of the developing blade and coming into contact or proximity to the photosensitive drum 21, to make the thin-layer developer adhere to the latent images formed on the photosensitive drum 21 and render visible the latent images formed on the photosensitive drum 21.
The developing roller is required to be well durable to the heat generated when electrified at a high voltage for a long time, not to cause contamination of the photosensitive drum 21 particularly because of decomposition of a high molecular material member, and not to cause changes in electrical resistance of the member (Japanese Patent No. 3186541). The developing roller may further undergo environmental changes in temperature, moisture and so forth to come to cause deflection of electrical resistance in a large width, and this may cause a great change in image density (Japanese Patent Application Laid-open No. H07-13415). Further, when an electrophotographic image forming apparatus is kept unoperated for a long period of time, the developing roller and a developer feed roller, both of which are at standstill, are brought into contact with each other for a long period of time. In such an occasion, any components bled out of the developer feed roller, hereinafter called “bleeding component”, may adhere to the surface of the developing roller, and such a developing roller may result in an undesirable “banding” or a density variation in the direction of drum rotation in an electrophotographic image. The banding is conspicuous in a halftone image or low-contrast image, and may remarkably occur when the electrophotographic image forming apparatus is kept unoperated under an environment of high-temperature and high-humidity, e.g., 40° C./95% RH over a long period of time.